Nonaqueous electrolytic solution secondary batteries such as lithium secondary batteries and the like are being put into practical use in broad applications covering so-called civilian applications for notebook-size personal computers and others as well as in-vehicle power sources for driving for automobiles and others and large-scale stationary power sources, etc. However, the recent requirements for technical advantages of nonaqueous electrolytic solution secondary batteries are being much higher and higher, and it is now desired to attain high-level battery characteristics such as high capacity, high output power, high-temperature storage stability, cycle characteristics, etc.
In particular, in case where lithium secondary batteries are used as a power source for electric vehicles, the lithium secondary batteries are required to have high output characteristics and input characteristics since electric vehicles need large energy at startup and acceleration and must efficiently regenerate the large energy generated during deceleration. In addition, since electric vehicles are used outdoors, in particular, lithium secondary batteries are further required to have high input-output characteristics (that is, the internal impedance of the batteries is low) at a low temperature of −30° C. or so, in order that such electric vehicles can be rapidly started up and accelerated even in cold months. Furthermore, even in repeated charge/discharge in high-temperature environments, the capacity reduction must be small and the internal impedance increase must be small.
Not limited to use for electric vehicles, in addition, in case where lithium secondary batteries are used also for various backup applications as well as for large-scale stationary power sources, for example, for power supply load leveling application, natural energy generation output stabilization application or the like, not only electric cells are large-sized but also a large number of electric cells are connected in series-parallel combination. Consequently, there may occur various problems of reliability and safety to be caused by various types of non-uniformity including fluctuation of discharge characteristics of individual cells, temperature fluctuation between different cells and fluctuation of capacity and charge state of individual cells. When cell planning and management are unsuitable, then there may occur some problems in that a part of cells that constitute an assembled battery may be kept in a highly-charged state or some cells may be kept in a high temperature owing to increase in the internal temperature thereof.
Specifically, current nonaqueous electrolytic solution secondary batteries are required to satisfy various requirements at an extremely high level that the initial capacity and input-output characteristics must be high, the internal impedance must be low, the capacity retention rate after durability tests such as high-temperature storage test and cycle test must be high and the input-output characteristics and the impedance characteristics must be kept excellent even after durability tests.
Heretofore, as a means for improving the characteristics of nonaqueous electrolytic solution secondary batteries, various techniques have been investigated. For example, Patent Document 1 says that using lithium fluorosulfonate as an electrolyte brings about a battery having a high discharge capacity in 60° C. charge/discharge cycle evaluation. According to Patent Document 1, when LiClO4 is used as the electrolyte, LiClO4 is decomposed owing to the electropositive potential of the positive electrode active material in the battery to form active oxygen therein, and the active oxygen attacks the solvent in the battery to accelerate the reaction of solvent decomposition. In addition, the document also says that, when CF3SO3Li, LiBF4 and LiPF6 are used as the electrolyte, the electrolyte decomposition is promoted owing to the electropositive potential of the positive electrode active material to form fluorine, and the fluorine attacks the solvent to accelerate the reaction of solvent decomposition.
Regarding the method for producing lithium fluorosulfonate, only two methods mentioned below have been reported (Non-Patent Document 1, Patent Document 2).
Non-Patent Document 1 reports that ammonium fluorosulfonate is mixed with an aqueous solution of lithium hydroxide to give lithium fluorosulfonate trihydrate.
However, in the method, after the ammonium salt has been once synthesized, the salt is again cation-exchanged into the lithium salt, and therefore the method is complicated and in addition, the method may have a trouble of contamination with the released ammonia.
In the document, in addition, potassium fluorosulfonate is said to be hydrolysable and the lithium salt may also have the possibility of hydrolysis, and therefore there still remains a problem that the hydrate could be stored stably for a long period of time.
Furthermore, when dissolved in an electrolytic solution, the crystallization water may have some negative influence of decomposing lithium hexafluorophosphate to produce, as a by-product, hydrogen fluoride, and therefore, the crystallization water must be previously removed therefore requiring further complicated operation.
Patent Document 2 describes the possibility of production of various lithium salts through salt exchange reaction between lithium chloride or lithium sulfate and various sodium salts/potassium salts in various solutions, including production of lithium fluorosulfonate. However, in Examples of the patent document, only lithium nitrate and lithium bromide that are stable in water are produced, but the document reports no example of producing lithium fluorosulfonate that is suspected of being hydrolyzable. In addition, in the patent document, for separation of the intended products of various lithium salts from the by-products of sodium or potassium chlorides or sulfates, the solubility difference therebetween is utilized. In the document, the solution is concentrated to thereby first precipitate the by-product that has a low solubility, and this is separated through filtration to take out the solution in which the intended product of various lithium salts is dissolved, thereby isolating the product. According to the method, a high recovery rate could not be asked for unless a solvent in which the difference in solubility between the intended product, lithium salt and the by-product salt is used, and the recovery rate in the method applied to production of lithium fluorosulfonate is unknown.
On the other hand, regarding salts with sodium or potassium that is the same alkali metal as lithium and that is more widely used than lithium in the art, the following production methods are known.
(1) A method of reacting sodium/potassium fluoride with sulfur trioxide or fuming sulfuric acid (Patent Documents 3, 4 and Non-Patent Document 2).
(2) Reaction of inorganic fluoride and sulfur trioxide (Non-Patent Reference 3 (hexafluorosilicate), Non-Patent Document 4 (hexafluorophosphate)).
(3) Salt-exchange reaction between fluorosulfonic acid and potassium acetate in acetic acid solvent (Non-Patent Document 5).